It is widely known that a soft handover, in which a frequency change does not occur, can be used generally for handover between base stations of one wireless communication service provider in a CDMA cellular system since all base stations of the one wireless communication service provider can use a same frequency for the soft handover.
The soft handover is a method for maintaining a communication link by simultaneously transceiving communication signal with both a source base station and a neighbor base station without changing a communication frequency when a mobile station is located at a cell boundary of the two base stations, i.e., when the mobile station moves from a coverage of the source base station to a coverage of the neighbor base station, and then disconnecting the communication link with the source base station, if a signal intensity of the source base station is weaken below a standard signal intensity as maintaining continuously the communication link with the neighbor base station. The above-mentioned soft handover provides the unstrained handover by eliminating an instant-disconnection, which is a chronic problem of an analog system, decreases a probability of call loss and maintains high quality communication.
However, the soft handover is not applicable in case a certain wireless communication service provider allocates different number of frequencies to the neighbor base stations according to a call density by considering an economy of a network design and an efficiency of investment, i.e., the neighbor base stations use different frequencies. That is, if the mobile station using a specific frequency of the source base station is moving to the cell of the neighbor base station, which does not equip the specific frequency, the soft handover cannot be applicable. Furthermore, the soft handover cannot be applicable between base stations possessed by two wireless communication providers using different frequencies although they adopt the same CDMA scheme. For these cases, the hardware handover has to be used.
The hard handover needs to be performed between frequencies in case of the handover from a wideband-CDMA (W-CDMA) time division duplex (TDD) to a W-CDMA frequency division duplex (FDD) or handover from global system for mobile communications (GSM) to the W-CDMA FDD.
Referring to FIG. 1, an example of the hard handover between a source base station 2 and a target base station 3 is explained as follows.
As shown in FIG. 1, the target base station 3 does not support communication frequencies f1 and f1′ of a current mobile station 1.
In FIG. 1, in case of the mobile station 1 having a dual-mode receiver, the mobile station 1 can measure a signal intensity of a new frequency f2 while demodulating downlink signals through the currently established frequency f1 and acquire synchronization of signals transmitted from the target base station 3.
Such the dual-mode receiver needs an additional hardware for radio frequency (RF) compared to a single-mode receiver and thus the complexity of a mobile device is increased.
To overcome above-mentioned problems, a compressed mode is defined in an asynchronous W-CDMA (FDD) standard (Release '99) of 3rd generation partnership project (3GPP), which was released at September 2000.
FIG. 2 illustrates an example of compressed mode transmission.
In the 3GPP standard, a frame has a length of 10 msec and consists of 15 slots.
For a transmission gap (TG) region 7 in a compressed frame, data transmission is not permitted. Instead of permitting the data transmission, a rate of frame errors of the compressed frame is maintained identical to that of a normal frame 5 by keeping a transmitting power at a slot region 6 in the compressed frame higher than a power of the normal frame 5.
The mobile station 1 having the single-mode receiver can search the signal intensity of the new frequency f2 in downlink on the handover situation shown in FIG. 1 by using the compressed mode of FIG. 2. That is, it is possible to search the signal intensity by dropping the current established communication frequency f1, changing to a frequency f2 and measuring the signal intensity of f2 in the TG region and after the TG region is over, demodulating the call channel of the frequency f1.
In the 3GPP (FDD) standard, the compressed mode is defined at not only the downlink but also the uplink. The downlink and uplink can be operated simultaneously as the compressed mode and only one of the downlink and the uplink can be operated as the compressed mode. A reason of defining the compressed mode in the uplink is for prevention of interference to the downlink when the mobile station 1 measures a frequency of the uplink and the downlink of a neighbor system such as 3GPP TDD or GSM. Therefore, even though the mobile station 1 employs the dual-mode receiver, the uplink needs to be operated as the compressed mode in case that the mobile station 1 measures the downlink of other system using a frequency similar to the frequency of the uplink.
In shortly, it is possible that the mobile station 1, which satisfies the 3GPP (FDD) standard, monitors a new frequency f2 of the downlink before disconnecting the current established call channel completely in the handover situation in FIG. 1 and call disconnection of the downlink can be avoided although there occurs the hard handover to the new frequency f2 since the synchronization of the downlink transmitted from the target base station 3 can be acquired by using a synchronization channel of f2 and a common pilot channel.
On the other hand, in case of the uplink, since the target base station receives no signal before the mobile station 1 drops the current established frequency f1′ and transmits signals by using a new frequency f2′, i.e., the hard handover occurs, the synchronization of the uplink needs to be started at the target base station 3 from a moment that the hard handover occurs. There occurs call disconnection since at least one frame is required to acquire the synchronization of the uplink even if an outperformed searcher is used in the target base station 3.
Moreover, since, according to the 3GPP W-CDMA (FDD) scheme, corresponding base stations operate in asynchronization, the target base station 3 cannot detect a round trip delay between the mobile station 1 and the target base station 3 and therefore, a time for acquiring synchronization in the target base station 3 may be more than several frames since a search window size becomes very large, which a searcher has to search, in case that a coverage area of the base station is huge. In this case, several frame disconnection may happen and current call disconnection also may be happened in more serious case. Also, in this case, a power may not be controlled properly, so that a capacity of the uplink of the target base station 3 may be incredibly decreased.
In the 3 GPP W-CDMA standard (Release'99), it is possible to perform the handover only in case a difference between a system frame number (SFN) of the target base station 3 and a connection frame number (CFN) of the mobile station 1 is known to the network. Therefore, the mobile station 1 needs to detect the SFN information of the target base station 3 by demodulating a common channel of the downlink of the target base station 3 before performing the handover and transmit the SFN information and a frame offset, which is the difference between the CFN of the mobile station 1, to the base station controller 4. Thereby allowing the base station controller 4 to decide an exact handover time, resulting in performing the handover. Abovementioned operations are well performed in the soft handover between same frequencies. However, in case of the hard handover between different frequencies, the mobile station 1 should use the compressed mode of the downlink for acquiring the SFN information of the target base station 3.
However, in the standard (Release'99), it is impossible to acquire the SFN information by using the compressed mode since at least 50 msec continuous demodulating time is required in the downlink for acquiring the SFN information. In case of the hard handover, since the mobile station 1 has to acquire the SFN information after being completely disconnected with the current established frequency and being connected to a new frequency, there may occur at least 50 msec additional call disconnection.
The above-mentioned problems are not limited to the inter-frequency hard handover in the W-CDMA FDD and they may happen when a multimode device having the dual-mode receiver or the single-mode receiver such as GSM/WCDMA FDD multimode device or W-CDMA TDD/W-CDMA FDD multimode device performs the hard handover from the GSM system to the W-CDMA FDD system or from the W-CDMA TDD system to the W-CDMA FDD system.
As mentioned above, the disconnection is inevitable for performing the inter-frequency hard handover defined in the 3GPP W-CDMA FDD standard. Specially, in case of the mobile station having the single mode receiver, since the compressed mode is used in the downlink for searching signals of the target base station so the frame offset between the target base station and the mobile station is not known to the network. In this case, the disconnection problem becomes more serious since at least 50 msec disconnection is generated during performing the inter-frequency handover. This is indicated as a problem in the 3GPP. Therefore, a handover method, which performs the inter-frequency hard handover without disconnection, is required for addressing the problems in the asynchronous W-CDMA standard.